DE102019135665B4 - Method for calibrating a position detection of a portable key element and access control system - Google Patents

Abstract

Method for calibrating a position detection of a portable key element (46) by an access control system (30) for a motor vehicle (32), the access control system (30) having a detection device (52) which is used to detect a position of the key element (46) relative to the Motor vehicle (32) is formed by radio, comprising the steps of:- generating a first trigger signal when the key element (46) is located in a first position relative to the motor vehicle (32), the respective trigger signal upon localization of the key element (46) is generated automatically in the first and/or a second position, the respective triggering signal being triggered by manually opening an access element (36, 38, 40, 42, 44) of the motor vehicle (32), starting an engine, sitting on a vehicle seat and/or is generated automatically by a sensor for detecting an area surrounding the motor vehicle (32),- detecting a first signal strength in response to the first trigger signal; and- determining a correlation of signal strength and distance of the key element (46) relative to the motor vehicle (32) as a function of the first signal strength at the first position.

Description

technical field

The present invention relates to a method for calibrating a position detection of a portable key element. Furthermore, the invention relates to an access control system for a motor vehicle.

State of the art

Automatically opening door systems are increasingly being installed in modern motor vehicles for reasons of comfort. For this, an owner receives a portable key element whose position can be detected by the motor vehicle. In response to certain detected positions, for example, a door can then be automatically unlocked and/or opened by the motor vehicle. Thus, a corresponding carrier of the key element does not first have to pick it up and/or operate it, for example by pressing a button, before access to the motor vehicle can be gained. This makes access to the motor vehicle particularly convenient.

In the US 2018/0234797 A1 describes a calibration of a machine learning algorithm for keyless entry.

In the U.S. 10,415,528 B2 describes a method of operating a Passive Access Passive Start (PEPS) system including calibration with respect to individual mobile wireless devices. The PEPS system is used in vehicles.

In the U.S. 10,123,297 B1 is a system, method and apparatus for locating a target device. In order to locate the target device, a calibration process must be performed by making a series of measurements with a host device receiving a wireless signal from a number of surrounding devices at points along a path

Provision can be made in a motor vehicle for access control to take place as a function of a position of a portable key element. The position of the key element is determined by radio. The position determination is usually based on a signal strength, which is correlated with a distance. However, due to manufacturing deviations and/or deviations in the respective electrical resistances, transmission power and/or reception quality can vary, as a result of which the position detection can be inaccurate. Provision can also be made to couple key elements of a different type to the access control of the motor vehicle for the first time, which could not yet be taken into account during the manufacture and configuration of the motor vehicle and the access control. Calibration can therefore be provided to reduce these effects and enable precise position detection.

Presentation of the invention

The object of the present invention is to increase the convenience of calibrating a position detection of portable key elements.

This object is solved by the subject matter of the independent patent claims which define the invention. Advantageous refinements and expedient developments of the invention are specified in the respective dependent claims.

A first aspect relates to a method for calibrating a position detection of a portable key element by an access control system for a motor vehicle. The access control system can have a detection device, which is designed to detect a position of the key element relative to the motor vehicle by radio. The motor vehicle can have at least one access element that can be automatically adjusted between a closed position and an open position, it being possible for the access element to be in the form of a door, for example. In the closed position, an access opening of the motor vehicle can be closed by the access element and/or the access element can be locked. In the open position, the access opening can be released by the access element and/or the access element can be unlocked so that it can be opened. The adjustment can take place automatically and/or automatically without the user actuating a switch. The adjustment can be, for example, pivoting, in particular of a door about a vertical pivot axis. The adjustment can additionally or alternatively be an unlocking or locking of a lock. The respective access elements can be adjusted, for example, by means of an actuator installed in the motor vehicle, which interacts with one or more access elements. For example, an electric motor can be provided for each vehicle door, by means of which the associated vehicle door can be pivoted between its respective positions and/or can be unlocked. The detection device can have, for example, multiple antennas for transmitting and/or receiving radio signals. A distance of the key element to a respective antenna can be, for example, in Depending on a signal strength of a radio signal received at the key element and/or the antenna of the detection device. Respective antennas of the detection device can be arranged on the motor vehicle, for example. For example, a radio signal can be triangulated for position detection. A position relative to the motor vehicle can be a distance and an angle in a horizontal plane relative to a point in the motor vehicle.

In order to calibrate the position detection, at least one position of the key element should be known, preferably at least two different positions, and respective assigned transmission powers should be determined. This makes it possible to correlate respective transmission powers with a distance, for example using a quadratic regression. A quadratic regression lends itself since a received signal strength decreases with the squared distance.

The method may include a step of generating a first trigger signal when the key element is located in a first position relative to the motor vehicle. The trigger signal can be triggered automatically or manually, for example. A manual triggering can take place, for example, by pressing a button on the key element when its carrier is in the first position. The first position can be specified for the wearer, for example at a distance of 3 m in front of the driver's door. With this procedure, the key element can be brought manually into the first position as a predetermined position. This specification can be displayed to the wearer, for example, on a screen of the key element after starting a calibration process. The trigger signal can be generated automatically, for example, by a sensor device whose sensor data allow a conclusion to be drawn about a position of the key element and thus its localization. For example, a camera of the motor vehicle can capture the key element and/or its carrier and the position can be determined by evaluating the image.

The method may include a step of detecting a first signal strength in response to the first trigger signal. For example, the detection device can be designed to measure the signal strength. A unit for the signal strength can be mW, for example. The signal strength is preferably registered for each antenna that registers a radio signal that is used to determine the position.

The method may include a step of determining a correlation of signal strength and distance of the key element relative to the motor vehicle as a function of the first signal strength at the first position. In the case of other signal strengths, the correlation allows a distance of the key element relative to the motor vehicle or respective antennas to be assigned to them.

The trigger signal allows a partially automated calibration to take place. In particular, the detection and/or measurement of signal strengths does not have to be laboriously done manually. If necessary, a user then does not have to ensure that the key element is positioned in the first position.

The signal strength can correspond to a strength of a radio signal between the key element and the detection device, measured at the receiver. The detection device is preferably arranged in the motor vehicle. The radio signal can be sent between the detection device and the key element.

For example, the key element may not be set up to interact with the access control system before it is calibrated. For example, a corresponding set-up, which can include a coupling with the detection device, can start the corresponding calibration. In this way, an exact position detection of new key elements and/or of key elements which differ from key elements previously interacting with the access control system in terms of structure, design and/or transmission power can be made possible simply and conveniently. The detection of a signal strength can be a measurement, for which a signal transmission is preferably caused, which takes place permanently and/or periodically. The first position can correspond to an area in an interior and/or an area surrounding the motor vehicle. For example, the environment can be divided into sectors, each of which corresponds to a position.

In a further embodiment of the method, it is provided that the method has a step of generating a second trigger signal when the key element is located in a second position relative to the motor vehicle. The method may include a step of detecting a second signal strength in response to the second trigger signal. The method can have a step of determining a correlation of signal strength and distance of the key element relative to the motor vehicle depending on the first signal strength at the first position and the second signal strength at the second position. Two signal strengths can significantly improve the accuracy of the correlation and thus also of the calibration become. Like the first signal strength, the second signal strength can be detected for all antennas. The second trigger signal can also be generated manually or automatically. The second position preferably differs from the first position. For example, the carrier of the key element can be specified to be in front of the rear door on the passenger side at a distance of 1 m when the second trigger signal is generated by pressing a button on the key element.

In a further configuration of the method, it is provided that the key element and/or the detection device has at least one first antenna and one second antenna, the signal strength being detected for each antenna and the correlation of signal strength and distance of the key element relative to the motor vehicle in Depending on the detected signal strength of the respective antennas. In this way, in particular, the accuracy of a triangulation can be improved. For example, the detection device can have at least three antennas for sending and/or receiving respective radio signals for the triangulation, for each of which an associated signal strength is detected. The positions of the respective antennas in the motor vehicle are preferably known, so that they can also be taken into account when determining the correlation between distance and signal strength.

In a further embodiment of the method, it is provided that the correlation of signal strength and distance of the key element relative to the motor vehicle is also dependent on radio shielding of the first and/or the second position, for example caused by a vehicle body and the respective arrangement of the antennas in the motor vehicle. is determined. The radio shielding can be, for example, a reduction in a received transmission power, which is not caused by its distance from the transmitter and the transmission power used there. The radio shielding can be taken into account in each case assigned to each antenna. A radio shielding strength can be determined as part of the method, for example with a simulation or an experiment. In a simulation, for example, respective CAD data of the motor vehicle can be used, in which case these can also have respective material data. The radio shielding can be specifically determined and/or taken into account for respective radio frequencies.

In a further refinement of the method, it is provided that the correlation determined is stored, in particular in a database device. The correlation can be called up when the position of the key element for access control is detected, for example by the detection device. The correlation can thus be made available to other access control systems, for example if a key element of the same type is to be linked to the access control of a motor vehicle of the same type. Then, if necessary, a further calibration can be dispensed with. The database device can be embodied locally, for example as part of the key element or of the motor vehicle. The database device can be designed centrally, for example as a server, which can receive and/or send the respective correlation data via a mobile radio network and/or the Internet. Respective correlations can be stored associated for an individual key element and/or for a key type. For example, respective correlations of the same key element can also be used when it is coupled to an access control system of another motor vehicle. The respective radio shieldings can also be stored and/or retrieved in the database device.

In a further refinement of the method, provision is made for BLE to be used as the radio standard. The Bluetooth low energy standard allows a particularly precise position determination with low power consumption. In particular, BLE is often already implemented on smartphones, so that a corresponding smartphone can simply be used as a key element. The different transmission power and/or the different structure of different smartphones, which can ultimately affect the received signal strength when determining the position and thus its accuracy, can be easily compensated for by the calibration.

According to the invention, it is provided that the respective triggering signal is generated automatically when the key element is located in the first and/or the second position. Automatic generation does not require any action by a user and is therefore particularly convenient. The method can thus also be carried out fully automatically and thus unnoticed by the carrier of the key element. An automatically generated trigger signal can also enable automatic intermittent recalibration, for example to be able to compensate for a degradation in transmission power and/or reception quality due to aging of components.

In a further embodiment of the method, it is provided that a sensor device relative to the position of the key element and/or a position of a carrier of the key element detected to the motor vehicle and the respective trigger signal is generated depending on the detected position of the key element and / or the position of the carrier. The position of the carrier can at least indirectly correspond to a position of the key element, so that the position of the carrier can be used as a proxy for the calibration. The position of the carrier can indirectly enable localization of the key element. The sensor device can have respective sensors for the detection, such as a door position sensor or a 3D camera.

According to the invention, it is provided that the respective triggering signal is generated by manually opening an access element of the motor vehicle, starting an engine, sitting on a vehicle seat and/or by a sensor for detecting an area surrounding the motor vehicle, in particular a camera or an ultrasonic sensor. is generated automatically. Manually opening an access element of the motor vehicle can be used to detect that the carrier of the key element and thus also the key element itself is in front of the access element. Accordingly, the position of the key element can be inferred from this. Sensors for monitoring access element positions are usually already present in motor vehicles, so that the automatic generation of the triggering signal is possible at low cost. For example, it is often already monitored whether a door of the motor vehicle is open or closed. Likewise, seat occupancy sensors are often already provided for seat belt warning devices. Since the wearer of the key element usually carries this in a trouser pocket, the position of the key element can also be inferred indirectly from this. Starting the engine also implies that the bearer of the key element is behind the wheel. The motor vehicle is preferably designed to allow the engine to be started without the key element being inserted into an ignition lock. The sensor for detecting an area around the vehicle can, for example, also be a sensor of a driver assistance system, such as a brake assistant, and/or can be used for autonomous driving. The sensor device can preferably be designed to detect a person in the surrounding area, it being possible for the person to be the carrier of the key element. The sensor for capturing the surrounding area can be embodied as a lidar or 3D camera, for example. By detecting the surrounding area, the position of the carrier of the key element can preferably be detected directly. For example, a driver assistance system often has to determine a distance from objects for its operation, as a result of which its respective sensors can already be designed for the corresponding detection.

The method is preferably designed to verify the position of the carrier of the key element and/or the key element detected with the sensor device, in particular by means of a detection by the detection device and/or sensor data of the sensor device. For example, the position can be recorded multiple times and the correlation can be determined as a function of the multiple positions. For example, a mean value of the positions can be used and/or respective extreme values can remain unconsidered. For this purpose, the method can also be applied iteratively and/or detected signal strengths can be stored. For example, when sitting in the driver's seat, the wearer may not always have the key element in his trouser pocket, but may also place it in a center console or on the front passenger seat. If the corresponding signal strength were used as a basis only once, this could result in an inaccurate calibration. With ten or more acquisitions, for example, such exceptions can have less impact on the calibration and it can therefore be sufficiently accurate.

The verification can, for example, take place alternatively or additionally by detecting at least one position of the key element relative to the motor vehicle by radio, with no correlation being used for this position detection. The detection can take place, for example, by means of the detection device. It is therefore an uncalibrated position detection by means of the detection device. This can be sufficiently accurate to determine, for example, whether the access element was opened by the bearer of the key element or by another person, such as a passenger or other attendant.

The verification can, for example, alternatively or additionally, also be based on a detection of whether more than one person is in the motor vehicle or has gotten into it. For example, two seat occupancy sensors can indicate an occupancy, which means that it can be assumed that two people have also got into the motor vehicle. In this case, for example, the correlation can be discarded in order to avoid unreliable indirect conclusions about the position of the key element by the sensor device.

The verification can, for example, alternatively or additionally, also be carried out using face recognition, in particular in the respective environmental data that is generated by the sensor device were recorded. For example, a 3D camera can record at least one image of the environment. Respective faces can be recognized with an image recognition algorithm and compared with faces stored in a database. For example, respective faces of authorized users of the motor vehicle can be stored in the database. If a person can be identified, it can be assumed, for example, that this person is the bearer of the key element. The position of this person can then be used as a basis for the calibration procedure. As a result, calibration can also be carried out reliably when a number of people have been detected in the vicinity of the motor vehicle.

In a further embodiment of the method, it is provided that the trigger signal is generated manually when the key element is located in the first and/or second position by actuating a trigger on the key element, such as a button. A particularly precise calibration can thereby be made possible. Optionally, the calibration process itself can be started by pressing the button. Respective required items can be indicated by an output device.

In a further embodiment of the method, it is provided that the method is designed to calibrate a position detection of at least one first and one second portable key element by an access control system for a motor vehicle, with an associated correlation of signal strength and distance of the key element relative to the Motor vehicle is determined. For this purpose, the respective signal strength detection assigned to each key element can take place at the respective positions. In this way, in particular, several different types of key elements can be calibrated. Once one of the two key elements has been calibrated, it can also be used to calibrate the other key element. For example, a key that is delivered with the motor vehicle can be calibrated and, with its position data, a smartphone that is to be subsequently coupled. The key supplied can also be calibrated at the factory. In both cases, its detected position can be used to verify the position of the other key element.

A second aspect relates to an access control system for a motor vehicle. The access control system can have at least one portable key element. The access control system can have a detection device which is designed to detect at least one position of the key element relative to the motor vehicle by radio. The access control system can be designed to calibrate the position detection of the portable key element. The access control system may include a trigger device for generating a first trigger signal when the key element is located in a first position relative to the motor vehicle. The detection device can be designed to detect a first signal strength in response to the first trigger signal and the access control system can have a correlation device for determining a correlation of signal strength and distance of the key element relative to the motor vehicle as a function of the first signal strength at the first position. In addition, the access control system can have an adjustment device for automatically adjusting at least one access element of the motor vehicle, such as a door, between its respective open position and respective closed position depending on the detected position of the key element relative to the motor vehicle.

The access control system can be designed to carry out the method according to the first aspect.

Advantageously, the detection device can have one or more transmitters and the key element can have respective corresponding receivers. Alternatively or additionally, the detection device can have one or more receivers and the key element can have the respective corresponding transmitter. If the transmitter is provided on the detection device or on the motor vehicle, the key element can be particularly simple and inexpensive. If the transmitter is provided on the key element, the motor vehicle or the detection device can be inexpensive, in particular when using the smartphone and/or the smartwatch as the key element. If both transmitters and receivers are provided on the detection device or on the motor vehicle, the system can work flexibly with a wide variety of key elements. If transmitters and receivers are provided on the key element and on the detection device or the motor vehicle, data can be exchanged, for example for authentication and/or unique identification of the respective key element. The multiple transmitters or receivers allow for easy triangulation to determine position. In the present case, corresponding can in particular mean that the respective transmitters and receivers are designed to use the same radio frequency and/or the same radio standard. For example, a receiver can be designed to to capture the radio signals from several transmitters, even at the same time. The individual signal strength of each received signal can also be recorded.

According to the invention, it is provided that the triggering device has a sensor device which automatically generates the triggering signal when the key element and/or a carrier of the key element is located in a first position relative to the motor vehicle. In particular, the sensor device can be designed to detect a manual opening of a door handle, a person in a seat and/or a person in the vicinity of the motor vehicle and generate the triggering signal as a function of this detection.

According to the invention, it is provided that the sensor device has a seat occupancy sensor, a door opening sensor, an engine start sensor and/or a sensor of a driver assistance system and/or autonomous driving system, such as a camera, radar, lidar or ultrasonic sensor. With these sensors, it is particularly easy to draw conclusions about the position of the key element, and the first trigger signal can thus be generated automatically.

In a further advantageous embodiment of the access control system, it can be provided that the key element is in the form of a radio key, smartphone, smartwatch, key card, RFID chip and/or remote control. In particular, a smartphone and a smartwatch are beneficial as key items since most people already own such a device and carry it with them all the time. This means that an additional device can be dispensed with. In addition, such devices often already have the necessary radio technology for position determination using the detection device. In addition, such devices often already have the computing power to implement parts of the access control system on them. Separate computing power does not have to be provided, which means that the access control system can be particularly cost-effective. However, there is such a large number of different smartphones and smartwatches that a factory calibration of the position detection in advance can be extremely time-consuming and the position detection without calibration can be inaccurate. The access control system allows easy and convenient calibration so that this can be compensated for. In particular, the positions of devices whose transmission power and/or reception quality of signals is unknown can also be precisely detected by the calibration.

Further features of the invention result from the claims, the respective exemplary embodiments and from the drawings. The features and combinations of features mentioned above in the description and the features and combinations of features mentioned below in the respective exemplary embodiments can be used not only in the combination specified in each case, but also in other combinations, without departing from the scope of the invention. The features and advantages resulting from the respective aspects also represent the respective features and advantages of other aspects.

character list

• 1 FIG. 12 shows a method for calibrating a position detection of a portable key element by an access control system for a motor vehicle in a flowchart.
• 2 shows a schematic view of an access control system for a motor vehicle.

Detailed Description of Embodiments

1 illustrates in a flowchart a method for calibrating a position detection of a portable key element 46. The method is executed by an access control system 30 for a motor vehicle 32, which in 2 is shown. The access control system 30 has a detection device 52 which is designed to detect a position of the key element 46 relative to the motor vehicle 32 by radio.

In the present case, the key element 46 is embodied as a smartphone and is carried by a person 48 . The motor vehicle 32 has a plurality of access elements 36, 38, 40, 42 designed as doors and an access element 44 designed as a trunk lid. The respective access elements can be adjusted between an open position and a closed position with the adjusting device 34 depending on a detected position of the key element 46 . For example, the person 48 with the key member 46 may approach the passenger door 38 as illustrated by arrow 58 . The access control system can then automatically open the passenger door 38 in response to the correspondingly detected positions.

For position detection, the detection device 52 can be designed, for example, to transmit a radio signal from the key element 46 to triangulate. The key element 46 has, for example, an antenna 50 with which a radio signal with a predetermined transmission power is generated. This signal is received by three spaced-apart antennas 54 of the detection device 52 . The detection device 52 is designed to determine the signal strength of the received signal assigned to each antenna 54 . With a known transmission power, the position of the key element 46 can then be determined on the basis of the different signal strengths at the antennas 54 and their known arrangement in the motor vehicle 32 .

However, the transmission power of the key element 46 can be unknown and/or can vary over its lifetime, for example due to aging. Alternatively or additionally, a reception quality at the antennas 54 can also vary, for example due to aging. The reception quality of the antennas 54 can also be unknown, for example due to manufacturing deviations. Reception quality can affect the detected signal strength. Overall, this can affect the accuracy of position detection via radio. In order to improve accuracy, the access control system is designed to use the method for calibrating the position detection according to FIG 1 to perform.

In a first step 10, the method provides for the generation of a first trigger signal when the key element 46 is located in a first position relative to the motor vehicle 32 . The localization of the key element in the first position is preferably inferred on the basis of respective sensor data from a sensor device 56 of the access control system 30 . The sensor device 56 can have a door sensor, for example, which is designed to detect an opening of an associated access element 36, 38, 40, 42 and/or an actuation of a door handle of this access element 36, 48, 40, 42. As a result, it can be concluded that the person 48 and thus also the key element 46 is in front of this access element 36, 38, 40 or 42 and that the key element 46 is therefore localized there. Alternatively or additionally, the sensor device 56 can have sensors of a driver assistance system, for example, with which the person 48 and/or the key element 46 and their position relative to the motor vehicle can be detected directly. Such a sensor can be a 3D camera, for example. The sensor device 56 can thus be designed as a triggering device which automatically generates the triggering signal when the key element 46 is located in the first position.

The position of the key element 46 determined in this way can be contained in the first trigger signal. Alternatively, the trigger signal may be generated only when the key member 46 is located in a predetermined first position, such as in front of a predetermined one of the respective access members 36, 48, 40, 42.

In a second step 12, the method provides for detecting a first signal strength in response to the first trigger signal. For example, the detection device 52 can measure and store an associated received first signal strength for each antenna 54 at the time of the triggering signal.

This means that the signal strength at a certain distance is known. In step 14, the method provides for determining a correlation of signal strength and distance of the key element 46 relative to the motor vehicle 32 as a function of the first signal strength at the first position. This correlation is the result of the calibration and can be used by the access control system 30 when determining future positions of the key element 46 relative to the motor vehicle 32, for example for automatic access control depending on the determined position of the key element 46. The detection device 52 can Determining the correlation and thus be designed as a correlation device.

Reference List

10

Step

12

Step

14

Step

30

access control system

32

motor vehicle

34

adjusting device

36

Access element / driver's door

38

Access element / passenger door

40

entry element

42

entry element

44

Access element / trunk lid

46

Key element / smartphone

48

person

50

antenna

52

detection device

54

antenna

56

sensor device

58

Arrow

Claims (11)

Method for calibrating a position detection of a portable key element (46) by an access control system (30) for a motor vehicle (32), the access control system (30) having a detection device (52) which is used to detect a position of the key element (46) relative to the Motor vehicle (32) is formed by radio, having the steps: - Generating a first trigger signal when the key element (46) is located in a first position relative to the motor vehicle (32), the respective trigger signal being generated automatically when the key element (46) is located in the first and/or a second position, wherein the respective trigger signal is generated by manual opening of an access element (36, 38, 40, 42, 44) of the motor vehicle (32), starting an engine, sitting on a vehicle seat and/or by a sensor for detecting an area surrounding the motor vehicle ( 32) is generated automatically, - detecting a first signal strength in response to the first trigger signal; and - Determining a correlation of signal strength and distance of the key element (46) relative to the motor vehicle (32) as a function of the first signal strength at the first position. procedure after claim 1 , the method further comprising the steps of: - generating a second trigger signal when the key element (46) is located in the second position relative to the motor vehicle (32); - detecting a second signal strength in response to the second trigger signal; and - determining a correlation of signal strength and distance of the key element (46) relative to the motor vehicle (32) as a function of the first signal strength at the first position and the second signal strength at the second position. Method according to one of the preceding claims, wherein the key element (46) and/or the detection device (52) comprises at least a first antenna (54) and a second antenna (54), the signal strength for each antenna (54) being detected and the Correlation of signal strength and distance of the key element (46) relative to the motor vehicle (32) as a function of the detected signal strength of the respective antennas (54). procedure after claim 3 , the correlation of signal strength and distance of the key element (46) relative to the motor vehicle (32) also depending on radio shielding of the first and/or the second position, for example caused by a vehicle body and the respective arrangement of the antennas (54) in the motor vehicle ( 32), is determined. Method according to one of the preceding claims, wherein the determined correlation is stored, in particular in a database device. Method according to one of the preceding claims, wherein BLE is used as the radio standard. Method according to one of the preceding claims, wherein a sensor device (56) detects the position of the key element (46) and/or a position of a carrier (48) of the key element (46) relative to the motor vehicle (32) and the respective trigger signal depending on the detected position of the key element (46) and / or the position of the carrier (46) is generated. A method according to any one of the preceding claims, wherein the trigger signal is manually generated upon location of the key member (46) in the first and/or second position by operating a trigger on the key member (46), such as a button. Method according to one of the preceding claims, wherein the method is designed to calibrate a position detection of at least one first and one second portable key element (46) by the access control system (30) for a motor vehicle, with an associated correlation of signal strength and Distance of the key element (46) is determined relative to the motor vehicle (32). Access control system (30) for a motor vehicle (32), comprising: - at least one portable key element (46); - A detection device (52) which is designed to detect at least one position of the key element (46) relative to the motor vehicle (32) by radio; and wherein the access control system (30) is designed to calibrate the position detection of the portable key element (46), wherein - the access control system (30) has a trigger device for generating a first trigger signal when the key element (46) is in a first position relative to the motor vehicle ( 32), wherein the triggering device comprises a sensor device (56) which is used to automatically generate the triggering signal when the key element (46) and/or a carrier (48) the key element (46) is located in a first position relative to the motor vehicle (32), the sensor device (56) being a seat occupancy sensor, a door opening sensor, an engine start sensor and/or a sensor of a driver assistance system and/or autonomous driving system, such as a camera, Radar, lidar or ultrasonic sensor. - the detection device (52) is designed to detect a first signal strength in response to the first trigger signal and - the access control system (30) has a correlation device for determining a correlation between signal strength and distance of the key element (46) relative to the motor vehicle (32) as a function from the first signal strength at the first position. Access control system (30) according to claim 10 , wherein the key element (46) is designed as a radio key, smartphone (46), smartwatch, key card, RFID chip and/or remote control.

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